1. Field of the Invention
The present invention generally relates to a liquid drop discharge apparatus for printing by discharging a discharge liquid (ink).
2. Description of the Background Art
In an ink jet discharge apparatus for printing by discharging liquid drops through nozzles, a print head and an ink tank are ordinarily connected by a flow path of exclusive use, so that the ink stored inside the ink tank is fed to the print head through such flow path.
The fed ink is pushed as an ink drop out through a nozzle hole by a pressure wave which is generated by a pressure generating device placed inside the print head. In an on-demand type ink jet head, the pressure generating device is, for instance, an actuator (such as a heater, a piezoelectric element or the like).
At this time, in order that the ink drops may be successfully discharged through the nozzle hole, a surface state of the ink including an ink meniscus at a portion of the nozzle hole, when the print head is not in operation, needs to be stably held. In order to maintain the meniscus, a force that counters a force of when the ink drop spontaneously falls by gravitation must be applied to the ink.
However, the above-mentioned print head of an on-demand type is equipped with a mechanism for ejecting liquid drop, but it is not at all provided with a mechanism for preventing the ink from leaking through the print head when the print head is not in operation. Therefore, a method in which a pressure which can prevent leakage of the ink is applied has been used. This pressure is called herein “backing pressure”.
However, the minimum control range of the backing pressure which is required in the ink jet discharge apparatus is around about 10 mm H2O or less, and besides the difference in pressure from an outside atmosphere is extremely small. Therefore, it is difficult in a system using a conventional vacuum pump or the like to control this pressure range with good precision.
As one of the conventional art techniques for providing the backing pressure, numerous examples in which a net-like porous body is used inside an ink tank are found (see, Patent Documents 1 to 3 mentioned below). It is a technique in which a capillary force, which is generated by fine pores of the porous body when the ink is sucked into the porous body, is utilized as the backing pressure for holding the ink; and it is possible to control the capillary force by dimensions, material, shapes or the like of the fine pores. In this method, however, there are resulting inconveniences such that the composition of the ink is changed due to adsorption of solid component of the ink into the porous body, or the selection of the material of the porous body become restricted by the resistance against the ink.
As another method for providing the backing pressure, there is also an example in which the above-mentioned porous body is not used (see, Patent Document 4 listed below). In this technique, a trap tube in a U-letter shape, communicating with the inside and the outside of the ink tank, is provided inside a substantially airtight ink tank, so that a liquid inside the tube flows to adjust the backing pressure as a pressure difference between the inside and the outside of the ink tank. Although this method is free from the above-explained inconveniences related to the porous body, it has a structure near hermetically-sealed state, so that this method cannot be applied to an apparatus structure in which an ink is fed into an ink tank from the outside.
See, Japan Patent Documents JP-B 2683187, JP-B 3513979, JP-A 2007-326303, and JP-B 4011560.